Redox-active guanidine ligands with pyridine and p-benzoquinone backbones

Article abstract of DOI:10.1002/ejic.201200679

Herein we report on the synthesis and some aspects of the chemistry of the new redox-active ligands 2,3,5,6-tetrakis(tetramethylguanidino)pyridine, 2,3,5,6-tetrakis(tetramethylguanidino)-p-benzoquinone and 2,3,5,6-tetrakis(N,N'- dimethyl-N,N'-ethyleneguanidino)-p-benzoquinone. All three ligands are strong electron donors. In addition, the p-benzoquinone systems feature low LUMO energies and small HOMO-LUMO gaps. A comparison of the ligands was made with regard to their optical properties, Bronsted basicity and electron-donor properties; results of quantum chemical calculations were included in this comparison. Subsequently, dinuclear copper complexes were prepared. Preliminary experiments on their redox chemistry followed. Copyright

Full text of DOI:10.1002/ejic.201200679

Job/Unit: I20679
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DOI: 10.1002/ejic.201200679
Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
Simone Stang,[a] Anna Lebkücher,[a] Petra Walter,[a] Elisabeth Kaifer,[a] and
Hans-Jörg Himmel*[a]
Keywords: Guanidine / Redox chemistry / Coordination compounds / Copper / Density functional calculations
Herein we report on the synthesis and some aspects of the
chemistry of the new redox-active ligands 2,3,5,6-tetrakis(te-
HOMO–LUMO gaps. A comparison of the ligands was made
with regard to their optical properties, Brønsted basicity and
electron-donor properties; results of quantum chemical cal-
culations were included in this comparison. Subsequently, di-
nuclear copper complexes were prepared. Preliminary ex-
tramethylguanidino)pyridine,
2,3,5,6-tetrakis(tetramethyl-
guanidino)-p-benzoquinone and 2,3,5,6-tetrakis(N,NЈ-di-
methyl-N,NЈ-ethyleneguanidino)-p-benzoquinone. All three
ligands are strong electron donors. In addition, the p-benzo- periments on their redox chemistry followed.
quinone systems feature low LUMO energies and small
[CuII|1a+·|CuII] and [CuII|1a2+|CuII] were synthesized and
Introduction
completely characterized.[13] Moreover, substitution of the
two aromatic protons in 1a by iodo or nitro groups (see
Scheme 1) was shown to affect the HOMO and, in particu-
lar, the LUMO energy, which offers the opportunity of tun-
ing the redox properties of the ligands.[15]
Aromatic compounds substituted with amino groups in
pairs in the para or ortho positions have been known as
electron donors for a long time. Examples include 1,4-di-
methylamino-benzene,[1] 1,2,4,5-tetrakis(dimethylamino)-
benzene[2] and 1,4,5,8-tetrakis(dimethylamino)naphthal-
ene.[3] The electron-donor strength can be further increased
by the use of guanidino groups in place of the amino
groups. Guanidines and guanidinates have been shown to
be versatile ligands,[4–9] which have found applications in
catalytic reactions such as lactide polymerization,[10] and in
the synthesis of complexes modeling enzymatic reactivity
such as the biological dioxygen reduction cascade.[11] The
compound 1,2,4,5-tetrakis(tetramethylguanidino)benzene
(1a) was the first member of a class of compounds named
GFA-n (guanidino-functionalized aromatic compounds; n
is the number of guandino groups in pairs in the para posi-
tions).[12] Because of the reduced steric demand of the gua-
nidino groups in the vicinity of the basic imino N atoms,
these compounds represent generally much better redox-
active ligands than 1,2,4,5-tetrakis(dimethylamino)benzene.
In the past, a number of dinuclear late-transition-metal
complexes of 1a was synthesized.[13,14] The chemistry of
copper complexes proved to be particular interesting, and
an almost complete charge-transfer series with examples for
the complex types [CuI|1a|CuI], [CuI|1a2+|CuI] (in the form
of a semiconducting coordination polymer), [CuII|1a|CuII],
Scheme 1. Structures of 1a and of the products of further aromatic
substitution, 1b and 1c.
In this work, we report on the synthesis and properties
of the new redox-active guanidine ligands 2,3,5,6-tetrakis-
(tetramethylguanidino)pyridine (2), 2,3,5,6-tetrakis(tetra-
methylguanidino)-p-benzoquinone (3) and 2,3,5,6-tetrakis-
(N,NЈ-dimethyl-N,NЈ-ethyleneguanidino)-p-benzoquinone
(4) shown in Scheme 2. These compounds are not just de-
rivatives of 1a; the additional function (pyridine N atom)
introduced in compound 2 promises new possibilities. Espe-
cially, the addition of RX (e.g. R = alkyl, X = halide) to
the [CuI|2|CuI] complexes might offer an access route to
[CuI|(2-R)+|CuI] complexes (2-Me denotes the Npyridine
-
[a] Anorganisch-Chemisches Institut, Ruprecht-Karls-Universität
Heidelberg,
Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax: +49-6221-545707
E-mail: hans-jorg.himmel@aci.uni-heidelberg.de
Homepage: www.uni-heidelberg.de/institute/fak12/AC/himmel/
index.htm
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200679.
Scheme 2. Structures of the new electron donors studied in this
work.
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alkylated compound 2). Moreover, oxidation of the cations starting with 2,3,5,6-tetrachloro-p-benzoquinone (see
(2-R)+ might lead to radical dications. Compounds 3 and 4 Scheme 4).[17,18] Reaction with 2-chloro-1,1Ј,3,3Ј-tetrameth-
have distinct electronic properties; substituted p-benzo- ylformamidinium chloride, [(Me2N)2CCl]Cl, then yielded
quinones are generally electron acceptors, and, therefore, 3 2,3,5,6-tetrakis(tetramethylguanidino)-p-benzoquinone (3).
and 4 should exhibit LUMOs of low energies.
In addition, the product of elimination of two dimethyl-
amino groups followed by cyclization was crystallized in
a small amount {2,6-bis(dimethylamino)benzo[1,2-d:4,5-dЈ]-
diimidazole-4,8(1H,5H)dione, see Supporting Information}.
A similar side-product was already isolated in the synthesis
of 1a.[15] Deep green crystals of 3 were obtained from
CH3CN. Its structure is shown in Figure 2. The C6 ring and
the two O atoms occupy one plane, in contrast to the situa-
tion found in tetrakis(dimethylamino)-p-benzoquinone.[19]
Hence, it could be concluded that the steric situation is re-
laxed. As anticipated, the C1–C2 bond [136.11(19) pm in 3]
is considerably shorter than the C1–C3Ј or C2–C3 bonds
[148.90(19) and 148.66(18) pm, respectively]. The variations
in the C–C bond lengths are comparable with those for re-
lated benzoquinones such as 2,3,5,6-tetrakis(dimethyl-
amino)-p-benzoquinone (with C–C bond lengths of 137,
146 and 151 pm).[19] The C=O bond length of 123.42(2) pm
in 3 also is close to that reported for 2,3,5,6-tetrakis(di-
methylamino)-p-benzoquinone.[19] By using a similar proto-
col as for the synthesis of 3, compound 4 was synthesized.
Crystals of this compound could be grown from CH3CN
solutions. Its molecular structure is also included in Fig-
ure 2. Again, the C6 ring and the O atoms are in the same
plane, which shows that the steric demand of the guanidino
groups is significantly smaller than that of the amino
groups in 2,3,5,6-tetrakis(dimethylamino)-p-benzoquin-
one.[19]
Results and Discussion
Ligand Synthesis and Properties
From the three new guanidine ligands described herein,
the synthesis of 2 is the most straightforward. It was pre-
pared in a two-step reaction according to Scheme 3. Re-
duction of 2,6-diamino-3,5-dinitropyridine (prepared by ni-
tration of 2,6-diaminopyridine)[16] with SnCl2/HCl led to
the protonated 2,3,5,6-tetraaminopyridine, which reacted
with the “Vilsmeier salt” 2-chloro-1,1Ј,3,3Ј-tetramethylfor-
mamidinium chloride in the presence of a base (NEt3) to
give the product in a 65% yield. Compound 2 can be crys-
tallized from Et2O solutions. Figure 1 shows its molecular
structure. The aromatic carbon atom (from the C–H group)
and the nitrogen atom of the pyridine ring could statistic-
ally change their positions in the crystal (with approxi-
mately equal probability for CH and N at both positions of
the ring, see N7 and C3 in Figure 1).
Scheme 3. Synthesis of 2.
Figure 1. Molecular structure of 2. Vibrational ellipsoids are drawn
at the 50% probability level. Hydrogen atoms are omitted for the
sake of clarity. Selected bond lengths (in pm) and bond angles (in
degrees): N1–C1 140.43(11), N1–C4 128.86(12), N2–C4 140.18(12),
N3–C4 137.72(13), N4–C2 140.99(11), N4–C9 129.95(12), N5–C9
137.32(11), N6–C9 139.08(13), N7–C1 136.77(12), N7–C2
137.11(12), C1–C2 141.25(12), C1–N1–C4 126.03(8), N2–C4–N3
113.17(8), C2–N4–C9 117.81(8), N5–C9–N6 114.07(8).
Scheme 4. Synthesis of 3 and 4.
The optical spectra of the guanidine electron donors have
been shown to change dramatically upon oxidation and are
For the synthesis of 3, the corresponding amine 2,3,5,6- valuable indicators for the oxidation state. The UV/Vis
tetrakis(amino)-p-benzoquinone (tabq) first had to be syn- spectra of 2 (see Supporting Information) contain two in-
thesized. This was accomplished in a two-step reaction tense bands, centred at λmax = 208 and 253 nm in CH3CN
2
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Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
is included in the Supporting Information. The absorption
spectrum is governed by two strong absorptions at λmax
=
226 and 360 nm. The emission spectrum features signals at
345 nm for excitation at 226 nm and 485 nm for excitation
at 360 nm. Hence, the spectra are significantly different to
those of 3. Quantum chemical calculations on 3 found no
energy minimum for a possible isomer with the quinol
structure. The push–pull type situation has to be taken into
account to explain the distinct optical properties of 3, and
an equilibrium between two isomers can be ruled out.
Figure 2. Molecular structure of (a) 3 and (b) 4. Hydrogen atoms
are omitted for the sake of clarity. Thermal ellipsoids are drawn at
the 50% probability level. Selected bond lengths (in pm) and bond
angles (in degrees) for 3: O1–C3 123.42(16), N1–C1 138.98(17),
N1–C4 130.27(17), N2–C4 137.31(18), N3–C4 137.45(18), N4–C2
139.12(17), N4–C9 129.72(18), N5–C9 138.27(19), N6–C9
137.18(17), C1–C2 136.11(19), C1–C3Ј 148.90(19), C2–C3
148.66(18), C1–N1–C4 122.49(12), C2–N4–C9 121.19(12), N2–C4–
N3 115.34(12), N5–C9–N6 115.51(12). Selected bond lengths (in
pm) and bond angles (in degrees) for 4: O1–C3 122.64(15), N1–C1
Figure 3. Absorption and emission spectra recorded for CH3CN
138.35(15), N1–C4 129.26(16), N2–C4 138.32(17), N3–C4 solutions of 3.
137.50(15), N4–C2 138.31(15), N4–C9 128.57(15), N5–C9
138.79(15), N6–C9 137.66(16), C1–C2 136.63(17), C1–C3Ј
149.32(16), C2–C3 149.12(17), C1–N1–C4 125.92(11), N2–C4–N3
108.13(10), C2–N4–C9 126.35(11), N5–C9–N6 108.11(10).
and 208 and 243 nm in Et2O, which can be assigned to tran-
Scheme 5. Example for an experimentally observed quinone–quinol
sitions localized at the pyridine ring. For comparison, three
absorption bands were reported for pyridine at λmax = 172,
194 and 248 nm.[20] The two bands at λmax = 331 and
360 nm in CH3CN and 323 and 358 nm in Et2O solution
can be assigned to transitions involving orbitals to which
the guanidino groups contribute. A corresponding band at
329 nm is observed in the case of 1a dissolved in CH3CN
solution.[12] CH3CN solutions of 3 are slightly fluorescent.
The emission spectrum presented in Figure 3 shows signals
at 286 and 560 nm. The absorption that corresponds to the
560 nm emission is virtually not visible in the UV/Vis spec-
trum. On the other hand, the 286 nm emission signal corre-
sponds to the 215 nm absorption, which translates into a
relatively large Stokes shift of 71 nm. Fluorescence in p-
benzoquinone derivatives is rare, but not unprecedented.
König observed fluorescence for the molecule 2-(β-hydroxy-
ethylamino)-p-benzoquinone, while other quinones such as
2-amino- and 2,5-diamino-p-benzoquinones are fluores-
cence-silent.[21] For this molecule, a quinone–quinol equilib-
rium was found in solution (see Scheme 5).[22] Therefore, we
synthesized this species again and compared its fluores-
cence with that of 3. We were, to the best of our knowledge
for the first time, able to crystallize the molecule in its qui-
equilibrium.
In Figure 4, the colours of CH3CN solutions of four dif-
ferent redox-active guanidine ligands are compared. While
compound 1a is colourless, derivative 2 is slightly yellow
and 3 has an intense green colour. The intense red colour
of 1c can also be seen.[15]
Figure 4. Photo showing the different colours of CH3CN solutions
of the four organic electron donors 1a, 2, 1c and 3.
In Figure 5 the energies of the HOMOs and LUMOs cal-
nol form from MeOH solution. A visualization of the struc- culated by DFT methods (B3LYP/6-311G**) are compared
ture alongside with a list of selected structural parameters for the three ligands 1a, 2 and 3. Moreover, the electron-
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density distribution in these two orbitals is visualized for 2
and 3. The HOMO in all cases is centred at the C6 ring. In
the case of 3, the orbitals of the LUMO have considerable
character from the C=O group (π* orbital), which leads to
a significant reduction of the LUMO energy. The HOMO–
LUMO gap shrinks from 4.4 and 4.3 eV for 1a and 2,
respectively, to only 2.4 eV for 3. For comparison, in com-
pound 1c, the gap is calculated to be 2.9 eV.[15] The experi-
mental and quantum chemical results show that the optical
properties could be varied over a large region, making these
ligands especially attractive for the synthesis of redox-active
complexes.
Figure 5. Comparison between the HOMO–LUMO energy gap cal-
culated (B3LYP/6-311G**) for the guanidine ligands 1a, 2 and 3.
Although the absolute LUMO energies cannot reliably be calcu-
lated by DFT methods, the energy differences can be treated with
more confidence.
In addition to the basic guanidino groups, the new com-
pounds 2–4 exhibit basic sites with the pyridine N atom in
2 and the oxygen atoms of the p-benzoquinone systems 3
and 4. Reaction of 2 with excess HCl resulted in tetrapro-
tonation. The salt [2H4]Cl4·2H2O could be crystallized from
hot CH3CN solutions (see the structure derived from XRD
in Figure 6a). Reaction of 3 with HCl afforded the salt
[3H4]Cl4·10H2O (see Figure 6b). In crystalline 3, a network
Figure 6. (a) Fraction of the crystal structure of [2H4]Cl4·2H2O.
Vibrational ellipsoids are drawn at the 50% probability level. Se-
lected bond lengths (in pm) and bond angles (in degrees): N1–C1
141.22(18), N1–C4 135.89(18), N2–C4 133.10(18), N3–C4
133.89(18), N4–C2 140.99(17), N4–C9 136.52(18), N5–C9
133.40(18), N6–C9 132.64(18), C1–C2 139.30(19), C1–N7/C3
135.64(18), C2–N7Ј/C3Ј 135.81(18), C1–N1–C4 122.21(11), N2–
C4–N3 121.23(13), C2–N4–C9 121.76(11), N5–C9–N6 121.36(13).
was established by hydrogen bonding involving the guanid- (b) Molecular structure of [3H4]Cl4·10H2O. Hydrogen atoms are
omitted for the sake of clarity. Thermal ellipsoids are drawn at the
50% probability level. Selected bond lengths (in pm) and bond
angles (in degrees): O1–C3 122.1(2), N1–C1 139.5(2), N1–C4
137.2(2), N2–C4 132.6(2), N3–C4 132.7(2), N4–C2 140.0(2), N4–
ine N–H, co-crystallized water molecules and the chloride
counterions.
While all four imino N atoms of the guanidino groups
were protonated, the pyridine N atom in 2 or the O atoms
in 3 were not attacked. Protonation is in both cases ac-
companied by an increase in the N=C bond length and a
decrease in the C–NMe2 single bond lengths. 2,3,5,6-Tetra-
kis(amino)-p-benzoquinones were shown to undergo dis-
tinct colour changes upon HCl addition.[23] For example,
C9 136.4(2), N5–C9 133.2(2), N6–C9 133.1(2), C1–C2 135.1(2),
C2–C3 148.1(2), C3–C1Ј 149.3(2), C1–N1–C4 125.29(15), N2–C4–
N3 122.16(16), C2–N4–C9 125.04(14), N5–C9–N6 121.66(15). The
network established through hydrogen bonding can be visualized
below.
2,3,5,6-tetrakis(dimethylamino)-p-benzoquinone,
which the pyridine nitrogen atom (in the case of 2) or the p-benzo-
gives black–brown DMF solutions, adopts first a deep blue quinone C=O group (in the case of 3). According to these
and then a yellow–red colour upon HCl addition. In the calculations, gas-phase protonation at the pyridine N atom
case of 3, the colour changes from deep green to red. Quan- in 2 is slightly favoured over protonation at the guanidino
tum chemical calculations (B3LYP/6-311G*) were carried groups (ΔG0 = –24 kJmol–1). In contrast, for 3, protonation
out to compare 2H+ and 3H+, in which protonation occurs at the carbonyl group is clearly disfavoured (ΔG0
=
at the imino N atom; tautomers arise from protonation at +48 kJmol–1) (see Scheme 6).
4
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Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
E1/2 (CH2Cl2) = 0.91 and 1.21 V vs. Ag/AgCl, which corre-
sponds to E1/2(CH2Cl2) = 0.45 and 0.74 V vs. Fc/Fc+. The
introduction of four highly basic guanidino functions turns
the p-benzoquinone system from an electron acceptor into
a relatively strong electron donor. Quantum chemical calcu-
lations on the thermodynamics of the gas-phase electron
transfer reaction between neutral 1a and the dication 32+
to give 1a2+ and 3 gave a ΔG0 value of –19.3 kJmol–1 (see
Scheme 6. Comparison between protonation at the guanidino
groups and the pyridine N atom in 2 or the O atom in 3.
Moreover, an empirical formula provided by Maksic´ et
al.[24] was employed to estimate the pK(BH+) value of 2H+
(protonation at the guanidino group) in CH3CN on the ba-
sis of quantum chemical calculations. A pK(BH+) value of
24.6 was obtained. In a similar way, the pK(BH+) value for
3H+ in CH3CN was estimated to be 23.9. For comparison,
a pK(BH+) value of 25.3 in CH3CN was derived for 1a.[25]
Thus, the compounds could be ordered with regard to their
Brønsted basicity as 3Ͻ 2Ͻ 1a.
Redox Chemistry
The electron donor capacity was then studied by cyclic
voltammetry (CV) (see Figure 7) and redox reactions. The
CV results are summarized in Table 1. The CV curve of 2
(Bu4NPF6 as electrolyte, scan speed 100 mVs–1) shows a
sharp reversible two-electron wave at E1/2(CH2Cl2) =
–0.30 V vs. Ag/AgCl or –0.76 V vs. Fc/Fc+ (Fc = ferrocene,
see Figure 7a). Virtually the same value was obtained for
1a.[12] Quantum chemical calculations (B3LYP/6-311G**)
gave a ΔG0 value of not more than +1.5 kJmol–1 for the
gas-phase electron transfer reaction between 1a and 22+
outlined in Scheme 7. Hence, the quantum chemical calcu-
lations are in line with the experimental results and predict
the donor capacity of 1a and 2 to be similar. In agreement
with this finding, the HOMO energies were calculated to be
similar (see Figure 5). A reversible one-electron wave, corre-
sponding to the oxidation to the trication, occurs at
E1/2(CH2Cl2) = 1.32 V vs. Ag/AgCl or 0.86 V vs. Fc/Fc+.
This value is clearly higher than that measured for 1a
(0.68 V, also vs. Fc/Fc+) (see Table 1). Figure 7b displays
the CV curve for 3 dissolved in CH2Cl2 (vs. Ag/AgCl), mea-
sured at a scan rate of 100 mV s–1 with Bu4NPF6 as electro-
lyte. A reduction wave was not observed. It presumably lies
below –1.5 V. For comparison, the E1/2(DMF) vs. SCE
value decreases from –0.40 V in p-benzoquinone to –1.07 V
in 2,3,5,6-tetrakis(dimethylamino)-p-benzoquinone.[19] The
LUMO energy of 3, although significantly lower than that
for 1a, is still too high for reduction to be observed within
the measured potential window. A relatively sharp revers-
ible two-electron oxidation wave appears at E1/2(CH2Cl2)
= –0.16 V vs. Ag/AgCl or –0.62 V vs. Fc/Fc+. Two further
Figure 7. CV curves (CH2Cl2, supporting electrolyte Bu4NPF6, ref-
erence electrode Ag/AgCl, scan rate 100 mVs–1) for (a) 2 (without
addition of ferrocene) and (b) 3 (without and with ferrocene added
as internal standard).
Table 1. Observed E values (in V, vs. Cp2Fe/Cp2Fe+) from CV
(CH2Cl2 solution).
1st wave
(two-electron)
2nd wave
(one-electron)
3rd wave
(one-electron)
Eox
Ered
E1/2
Eox Ered E1/2 Eox Ered E1/2
1a
2
3
–0.71 –0.82 –0.76 0.72 0.65 0.68
–0.65 –0.85 –0.76 0.92 0.80 0.86
–0.51 –0.73 –0.62 0.47 0.39 0.43 0.78 0.70 0.74
–
–
–
–
–
–
reversible one-electron oxidation waves were observed at Scheme 7.
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Scheme 7). Hence the calculations are again in line with the
CV results, which indicate that 3 is a slightly weaker elec-
tron donor than 1a. Consequently, the HOMO energy is
lower by 0.28 eV for 3 than for 1a (see Figure 5).
The guanidine groups could also be oxidized by chemical
means. We were especially interested in obtaining infor-
mation about the structure and optical properties of the
oxidized free ligands, which can then be compared with the
data observed after oxidation of the dinuclear (copper)
complexes. In the case of the reaction between 2 and Br2,
oxidation was accompanied by aromatic substitution (see
Scheme 8). The proton released in this process bonds to one
of the imino N atoms. The salt [(2-Br)H](Br3)3 crystallizes
upon addition of toluene to the CH3CN solution. Figure 8a
shows its structure, and selected structural parameters can
be found in the figure caption. On the basis of these param-
eters, the dication can be best described as a pair of bis-
guanidino-allyl cations connected by two C–C single bonds
[C1–C2 and C4–C5, with bond lengths of 149.2(4) and
151.4(4) pm, respectively]. In a similar way, reaction with I2
resulted in oxidation and substitution. In this case, the salts
(2-I)(I3)I and (2-I)I2 were isolated from the reaction mixture
(see Figure 9 and Supporting Information). Protonation
was not observed in this case.
Figure 8. Fractions of the crystal structures of (a) {[2-Br]H}-
(Br3)3 and (b) [2-I](I3)I. Vibrational ellipsoids are drawn at the 50%
probability level. Selected bond lengths (in pm) and bond angles
(in degrees) for {[2-Br]H}(Br3)3: Br1–C3 188.3(3), N1–C1 129.9(3),
N1–C6 137.0(4), N2–C6 132.8(4), N3–C6 132.8(4), N4–C2
137.7(3), N4–C11 138.9(4), N5–C11 132.0(4), N6–C11 131.6(4),
N7–C4 128.0(4), N7–C16 138.5(4), N8–C16 132.2(4), N9–C16
134.0(4), N10–C5 130.1(4), N10-C21 138.7(4), N11-C21 133.2(4),
N12-C21 132.5(4), N13–C1 133.9(3), N13–C5 134.6(3), C1–C2
149.2(4), C2–C3 134.9(4), C3–C4 145.4(4), C4–C5 151.4(4), C1–
N1–C6 121.1(2), N2–C6–N3 122.4(3), C2–N4–C11 131.9(2), N5–
C11–N6 123.2(3), C4–N7–C16 126.4(2), N8–C16–N9 122.3(3), C5–
N10–C21 117.3(2), N11–C21–N12 123.4(3). Selected bond lengths
(in pm) and bond angles (in degrees) for [2-I](I3)I: I2–I4 290.75(8),
I4–I5 298.19(9), I1–C3 209.4(4), N1–C1 132.5(6), N1–C6 136.3(6),
N2–C6 133.2(6), N3–C6 135.1(6), N4–C11 136.1(6), N5–C11
133.7(7), N6–C11 132.9(6), N7–C4 131.0(6), N7–C16 134.1(6), N8–
C16 132.7(6), N9–C16 135.5(7), N10–C5 129.7(6), N10-C21
137.5(6), N11-C21 133.4(6), N12-C21 134.1(6), N13–C1 133.7(6),
N13–C5 135.0(6), C1–C2 151.8(6), C2–C3 141.0(6), C3–C4
137.2(6), C4–C5 153.1(6), I2–I4–I5 172.896(18), C1–N1–C6
120.0(4), C2–N4–C11 129.0(4), C4–N7–C16 128.2(4), C5–N10–
C21 119.2(4), N2–C6–N3 120.6(4), N5–C11–N6 120.7(4), N8–
C16–N9 118.8(4), N11–C21–N12 120.7(4).
Scheme 8. Oxidation of 2 with Br2.
Besides the structural information, the absorption spec-
tra are characteristic of the oxidized ligands. The spectra of
the free ligands could serve as reference for the analysis of
the electronic properties in oxidized dinuclear complexes.
The two-electron oxidation of 2 was accompanied by a col-
our change of the solution from pale yellow to deep red.
Figure 9 shows as an example the UV/Vis spectrum of solu-
tions of (2-I)(I3)I. The absorption maxima were detected at
245, 290, 366 and 513 nm. The broad 366 nm band appears
to be composed of two features; a shoulder is marginally
visible at 390 nm. The band at 290 nm together with one of
–
the features of the 366 nm band can be assigned to the I3
anion [cf. 292 and 363 nm reported if the anion was formed
by reaction between I2 and (n-C4H9)4NI in CH3CN[26]].
Oxidation of 3 with I2 in a CH2Cl2 solution furnished
the salt (3)I2. Crystals were obtained directly from the solu-
tion (see Figures 10 and 11a). Most remarkably, the C=C C1 and N4–C2, see Figure 11) decrease by more than 8 pm
bond length of 136.11(2) pm (C1–C2) in neutral 3 increases [138.98(2) and 139.12(2) pm before and 130.6(4) and
by more than 9 ppm [to 145.7(4) pm] upon two-electron 130.5(4) pm after oxidation]. In the same way, the salt (4)I2
oxidation. At the same time, the N=C bond lengths of could be prepared by reaction of 4 with I2 (see Figure 11b).
130.27(2) (N1–C4) and 129.72(2) pm (N4–C9) in neutral 3 Solutions of (3)I2 and (4)I2 are deep blue or purple. The
increase to 136.5(4) and 136.7(4) pm, respectively. In con- UV/Vis spectra of neutral and dicationic 3 are compared in
trast, the bond lengths between the N atom of the guan- Figure 10. The spectrum of (3)I2 shows a strong and broad
idino group and the C atom of the central C6 ring (N1– absorption at 581 nm.
6
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Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
Figure 9. UV/Vis spectrum of [2-I](I3)I dissolved in Et2O.
Figure 11. Fraction of the crystal structures of (a) (3)I2 and (b) (4)
I2. Vibrational ellipsoids are drawn at the 50% probability level.
Hydrogen atoms are omitted for the sake of clarity. Selected bond
lengths (in pm) and bond angles (in degrees) for (3)I2: O1–C3
124.4(3), N1–C1 130.6(4), N1–C4 136.5(4), N2–C4 133.2(4), N3–
C4 134.5(4), N4–C2 130.5(4), N4–C9 136.7(4), N5–C9 133.0(4),
N6–C9 134.2(4), C1–C2 145.7(4), C1–C3Ј 146.0(4), C2–C3
146.8(4), C1–N1–C4 124.9(3), N2–C4–N3 120.5(3), C2–N4–C9
125.3(3), N5–C9–N6 121.2(3). Selected bond lengths (in pm) and
bond angles (in degrees) for (4)I2: O1–C3 124.0(2), N1–C1 130.5(2),
N1–C4 135.6(2), N2–C4 133.4(3), N3–C4 134.2(2), N4–C2
130.1(2), N4–C9 136.1(2), N5–C9 132.9(2), N6–C9 133.7(2), C1–
C2 145.6(2), C1–C3 146.8(2), C1–N1–C4 125.44(16), N2–C4–N3
111.39(16), C2–N4–C9 127.02(16), N5–C9–N6 111.68(16).
Figure 10. Comparison between the UV/Vis spectra of neutral 3
and (3)I2.
Coordination Compounds
Reaction between 2 and 2 equiv. CuBr or CuI furnished
the dinuclear complexes [2(CuBr)2] and [2(CuI)2]
(Scheme 9). Their structures are presented in Figure 12. All
bonding parameters indicate a strong ligand–metal bond-
ing. The imino C=N bonds of the guanidino groups
[131.9(4) and 132.7(4) pm] are elongated with respect to
those of free 2 [128.86(12) and 129.95(12) pm], which indi-
cates the presence of ligand–metal π bonding in addition to
Scheme 9. Reaction of 2 with CuI compounds.
σ bonding.[27] Because of the strong Lewis basicity of 2, the tion. Crystals of this compound were grown from acetoni-
copper metal is “satisfied” with a relatively low coordina- trile solutions. Unfortunately, the quality was not high
tion number of 3. The geometry of the complexes is only enough for a detailed structural analysis. Nevertheless, pre-
slightly distorted – the two halide-Cu–N angles and Cu–N liminary analysis indicate that the compound consists of 1D
bond lengths are slightly different.
cationic chains, in which the Cu atoms are tetra-coordi-
In addition, we reacted 2 with 2 equiv. CuCN. Again, nated and connected together by two iodo bridges. The
complex formation proved possible, and yellow crystals of complex [2(CuI)2] did not react with CH3I both at room
the dinuclear complex [2(CuCN)2] were grown from temperature and at 48 °C (Scheme 10).
CH3CN solutions. Figure 13 displays its molecular struc-
Reaction between 3 and CuI resulted in a product mix-
ture, which is similar to those of [2(CuBr)2] and [2(CuI)2].
ture, from which a small amount of black crystalline mate-
These three complexes of the type [CuI|2|CuI] could serve rial was isolated; structural analysis confirms a 1D coordi-
as starting points for oxidation experiments aiming at the nation polymer [3(CuI)(CuI2)I]n (see Figure 14), in which
synthesis of an electron transfer series. First, we reacted the copper atoms are connected by one iodo bridge. The
[2(CuI)2] with I2. The product is the coordination polymer structural data indicate that guanidine 3 is twofold oxidized
{[(2-I)(CuI)2](I3)2}n, which represents an example for the and coordinated by CuI. Hence, this compound represents
type [CuI|22+|CuI]. As observed in the experiment with the the first coordination compound of the general type
free ligand, oxidation is accompanied by aromatic substitu- [CuI| 32+|CuI].
Eur. J. Inorg. Chem. 0000, 0–0
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S. Stang, A. Lebkücher, P. Walter, E. Kaifer, H.-J. Himmel
FULL PAPER
Scheme 10. Experiments to probe the reactivity of [2(CuI)2].
Figure 12. Molecular structures of (a) [2(CuBr)2] and (b)
[2(CuI)2]. Vibrational ellipsoids are drawn at the 50% probability
level. Hydrogen atoms are omitted for the sake of clarity. Selected
bond lengths (in pm) and bond angles (in degrees) for [2(CuBr)2]:
Cu1–Br1 228.76(9), Cu1–N1 201.8(2), Cu1–N4 203.1(2), N1–C1
139.6(3), N1–C4 132.7(4), N2–C4 135.3(4), N3–C4 135.5(4), N4–
C2 140.7(4), N4–C9 131.9(4), N5–C9 136.1(4), N6–C9 135.8(4),
N7–C1 135.4(3), C1–C2 140.3(4), C2–C3 138.2(3), N1–Cu1–N4
83.12(10), N1–Cu1–Br1 136.13(7), N4–Cu1–Br1 140.73(7). Se-
lected bond lengths (in pm) and bond angles (in degrees) for
[2(CuI)2]: Cu1–I1 244.58(10), Cu1–N1 201.0(2), Cu1–N4 203.3(2),
N1–C1 140.1(3), N1–C4 133.0(3), N2–C4 135.4(4), N3–C4
135.6(4), N4–C2 141.7(3), N4–C9 131.4(3), N5–C9 137.0(4), N6–
C9 137.0(4), N7–C1 134.3(3), C1–C2 141.0(4), C2–C3 137.7(3),
N1–Cu1–N4 83.69(9), N1–Cu1–I1 137.94(6), N4–Cu1–I1
138.36(6).
Figure 14. Part of the structure of the polymer [3(CuI)(CuI2)I]n
from two perspectives. Hydrogen atoms are omitted for the sake of
clarity. Selected bond lengths (in pm) and bond angles (in degrees):
Cu1–I1 255.06(13), Cu1–I2 260.25(13), Cu1–N1 209.1(6), Cu1–N4
211.4(6), Cu2–I2 258.64(15), Cu2–I3 254.76(12), Cu2–N7 211.3(6),
Cu2–N10 208.7(6), N1–C1 129.5(9), N1–C4 138.8(9), N2–C4
133.1(9), N3–C4 133.9(9), N4–C2 133.4(9), N4–C9 138.3(9), N5–
C9 131.1(9), N6–C9 132.1(10), C1–C2 145.8(10), C1–C3 145.3(9),
N1–Cu1–N4 78.2(2), I1–Cu1–I2 113.50(5), I2–Cu2–I3 116.01(4),
N7–Cu2–N10 77.0(2).
Conclusions
In this work, we reported the synthesis of the new redox-
active ligands 2,3,5,6-tetrakis(tetramethylguanidino)pyr-
idine (2), 2,3,5,6-tetrakis(tetramethylguanidino)-p-benzo-
quinone (3) and 2,3,5,6-tetrakis(N,NЈ-dimethyl-N,NЈ-ethyl-
ene-guanidino)-p-benzoquinone (4). Compounds 3 and 4
feature high HOMO and low LUMO energies, which result
in small HOMO–LUMO gaps and intensely green solu-
tions. The properties and reactivity were discussed on the
basis of experimental results as well as quantum chemical
(DFT) calculations. The discussion especially concentrated
on the optical properties, Brønsted basicity and electron do-
nor capacity. A comparison between the ligands before and
Figure 13. Molecular structure of [2(CuCN)2]. Vibrational ellip-
soids are drawn at the 50% probability level. Hydrogen atoms are
omitted for the sake of clarity. Selected bond lengths (in pm) and
bond angles (in degrees): Cu–N1 202.88(19), Cu–N4 202.74(19),
Cu–C14 186.1(3), N1–C1 140.4(3), N1–C4 132.9(3), N2–C4
135.7(3), N3–C4 135.7(3), N4–C9 132.2(3), N5–C9 136.8(3), N6– after oxidation is of importance to understand the elec-
C9 136.1(3), N7–C1 134.3(3), N8–C14 115.5(3), C1–C2 141.0(3),
tronic properties of their complexes. Dinuclear copper com-
C2–C3 138.3(3), N1–Cu–N4 82.34(8), N1–Cu–C14 134.69(10),
plexes were then synthesized. In preliminary experiments
N4–Cu–C14 142.96(10).
on the redox chemistry of these complexes, we oxidized the
8
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Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
ppm. 13C{1H} NMR (100.56 MHz, CD3CN), HSQC, HMBC: δ =
complex [2(CuI)2] with I2 to give the coordination polymer
{[(2-I)(CuI)2](I3)2}n with dicationic ligand units. Reaction
of 3 with CuI leads directly to a 1D coordination polymer
[3(CuI)(CuI2)I]n with oxidized ligand units.
160.16 (s, NCN), 158.94 (s, NCN), 148.55 (s, CAr), 133.45 (s, CAr),
123.17 (d, CAr, CH), 39.96 (q, CH3) ppm. MS (HR-EI+): m/z (%)
= 531.4240 (100) [M]+(calcd. 531.4233). IR (CsI): ν = 2997 (w),
˜
2931 (w), 2885 (w), 2804 (w), 1608 (vs), 1562 (vs), 1508 (s), 1458
(s), 1419 (s), 1377 (s), 1141 (s), 1018 (m), 894 (w), 725 (w), 671
(w) cm–1. UV/Vis (CH3CN): λ (ε, Lmol–1 cm–1) = 360 (11050), 331
(11047), 253 (19906), 208 (41062) nm. UV/Vis (Et2O): λ (ε,
Lmol–1 cm–1) = 358 (17142), 323 (13295), 243 (36855), 208 (59200)
Experimental Section
All synthetic work was carried out under an inert gas atmosphere
by using standard Schlenk techniques. All solvents were rigorously
dried prior to their use. NMR spectra were measured on a DRX-
200, Bruker Avance II 400 or a Bruker Avance III 600 spectrometer.
A Cary 5000 spectrophotometer was used for UV/Vis spectroscopy.
For IR spectroscopy, CsI discs of the compounds were measured
with a FTIR Biorad Merlin Excalibur FT 3000 spectrometer. Ele-
mental analyses were carried out at the Microanalytical Laboratory
of the University of Heidelberg. An EG&G Princeton 273 appara-
tus was used for the CV measurements. The curves were recorded
at different scan rates in the range 50–200 mVs–1 with an Ag/AgCl
electrode as reference electrode and Bu4NPF6 (Fluka, electrochemi-
cal grade) as electrolyte. HR-EI spectra were recorded on a JEOL
JMS-700 magnetic sector, ESI and HR-ESI spectra on a Finnigan
LCQ quadrupole ion trap and FAB spectra were recorded on a
MAT 8230. 2,6-Diamino-3,5-dinitropyridine[16] and 2,3,5,6-tetra-
aminobenzoquinone[17,18] were synthesized according to reported
procedures. All other reagents were of reagent-grade quality as ob-
tained from commercial suppliers and were used without further
purification.
nm.
Crystal
data
for
C25H49N13:
Mr
=
531.77,
0.40ϫ0.40ϫ0.35 mm, monoclinic, space group P21/n,
a
=
9.892(2), b = 11.025(2), c = 13.816(3) Å, β = 97.39(3), V =
1494.3(5) Å3, Z = 2, dcalcd. = 1.182 Mgm–3, Mo-Kα radiation
(graphite monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.37
to 33.16°. Reflections measured 11249, independent 5662, Rint
0.0317. Final R indices [IϾ2σ(I)]: R1 = 0.0466, wR2 = 0.1153.
=
2,3,5,6-Tetrakis(tetramethylguanidino)-p-benzoquinone (3): Oxalyl
chloride (2.075 mL, 3.019 g, 23.79 mmol) was added dropwise to
a solution of tetramethyl urea (0.571 mL, 0.553 g, 4.758 mmol) in
CHCl3 (15 mL). Subsequently, the reaction mixture was stirred for
16 h under reflux. The solvent was then removed under vacuum.
The remaining solid was washed three times with 10 mL portions
of Et2O, dried and dissolved in CH3CN (12 mL). This solution was
slowly added at a temperature of 0 °C to a suspension of 2,3,5,6-
tetraaminobenzoquinone (0.200 g, 1.199 mmol) in 12 mL of
CH3CN and NEt3 (1.989 mL, 1.444 g, 14.27 mmol). The suspen-
sion was gradually warmed from 0 °C to room temperature (over
a period of 4.5 h). Subsequently, the solvent was removed under
vacuum, which left a green–grey solid residue. This residue was
dissolved in an HCl solution (10 mL of a 10% solution), which led
to a dark red solution. After addition of a NaOH solution (10 mL
of a 25% solution), the solution turned dark green. The solution
was then extracted several times with Et2O, until the Et2O phase
remained colourless. The combined organic phases were dried with
K2CO3, and the solvent then removed under vacuum. Yield:
90.30 mg (0.16 mmol, 14%) in the form of a green solid recrys-
tallized from CH3CN. C26H48N12O2 (560.76): calcd. C 55.69, H
8.63, N 29.97; found C 55.44, H 8.63, N 28.32. 1H NMR
(600.13 MHz, CD3CN): δ = 2.69 (s, CH3) ppm. 13C{1H} NMR
(150.90 MHz, CD3CN), HSQC, HMBC: δ = 183.26 (s, C=O),
161.87 (s, NCN), 135.69 (s, Cring), 39.40 (q, CH3) ppm. MS (HR-
2,3,5,6-Tetraaminopyridine (tap): A suspension of 2,6-diamino-3,5-
dinitro-pyridine (2.00 g, 10.05 mmol) in EtOH (75 mL) was added
to a solution of SnCl2 (38.11 g, 200.9 mmol) in concentrated HCl
(100 mL). The suspension was stirred for 2.5 h at a temperature
of 40 °C. The reaction mixture was then allowed to cool to room
temperature. A pale yellow solid precipitated. The solution was re-
moved by filtration, and the solid washed several times with EtOH
to yield tap·2SnCl2·3HCl (4.65 g, 7.43 mmol, 74%). 1H NMR
(199.92 MHz, DMSO): δ = 7.80–8.80 (br. s, 8 H, NH2), 7.43 (s, 1
H, HAr) ppm. MS(EI+): m/z (%) = 139.1 (97.08) [tap]+, 124.2
(100.0) [tap-NH2]+. C5H12N5·2SnCl2·3HCl (627.77): calcd. C 9.57,
H 1.93, N 11.16; found C 9.66, H 2.07, N 10.82.
2,3,5,6-Tetrakis(tetramethylguanidino)pyridine (2): N,N,NЈ,NЈ-
Tetramethylurea (3.8 mL, 3.68 g, 31.66 mmol) was dissolved in
CHCl3 (25 mL). After dropwise addition of oxalyl chloride
(13.90 mL, 20.22 g, 159.3 mmol), the reaction mixture was stirred
for 16 h under reflux. The solution was then cooled back to room
temperature, and the solvent removed under vacuum. The remain-
ing solid was washed three times with 15 mL portions of Et2O and
dried under vacuum. Subsequently, the colourless solid was redis-
solved in CH3CN (30 mL). This solution was added slowly to a
suspension of tap·2SnCl2·3HCl (2.199 g, 3.51 mmol) in CH3CN
(40 mL) and NEt3 (10 mL) kept at a temperature of 0 °C. An
orange suspension was formed which was stirred for 4 h at 0 °C.
The orange solution was removed by a cannula, and the remaining
orange solid washed three times with 10 mL portions of CH3CN. It
was then dissolved in a 10% HCl solution (10 mL). Subsequently, a
25% NaOH solution (20 mL) was added, and the reaction mixture
extracted with Et2O (5ϫ100 mL). The combined organic phases
were dried with K2CO3, and the solvent removed with a rotary
evaporator to yield 2 (1.21 g, 2.27 mmol, 65%) as a pale green so-
lid. Recrystallization from Et2O afforded crystals suitable for XRD
analysis. C25H49N13 (531.74): calcd. C 56.47, H 9.29, N 34.24;
EI+): m/z (%) = 560.4022 (100) [M]+ (calcd. 560.4023). IR (CsI): ν
˜
= 2997 (w), 2932 (m), 2882 (m), 2801 (w), 1597 (s), 1562 (vs), 1508
(s), 1458 (m), 1423 (m), 1381 (s), 1346 (m), 1285 (m), 1223 (s), 1142
(s), 1107 (w), 1049 (m), 988 (s), 976 (s), 914 (m), 806 (w), 760 (m),
733 (w), 710 (w), 671 (w), 640 (w), 563 (w), 552 (w), 525 (w) cm–1.
UV/Vis (CH3CN): λ (ε, Lmol–1 cm–1) = 398 (13919), 286 (21840),
215 (45746) nm. Crystal data for C26H48N12O2: Mr = 560.76,
0.35ϫ0.32ϫ0.25 mm, monoclinic, space group C2/c,
a =
24.037(5) Å, b = 12.855(3) Å, c = 10.874(2) Å, β = 113.58(3)°, V
= 3079.5(13) Å3, Z = 4, dcalcd. = 1.209 Mgm–3, Mo-Kα radiation
(graphite monochromated, λ = 0.71073 Å), T = 100 K, θrange 1.83
to 32.05°. Reflections measured 10283, independent 5331, Rint
0.0514. Final R indices [IϾ2σ(I)]: R1 = 0.0557, wR2 = 0.1353.
=
2,3,5,6-Tetrakis(N,NЈ-dimethyl-N,NЈ-ethylene-guanidino)-p-benzo-
quinone (4): Oxalyl chloride (1.297 mL, 1.887 g, 14.87 mmol) was
dropwise added to a solution of 1,3-dimethylimidazolidine-2-one
(0.321 mL, 0.339 g, 2.974 mmol) in CHCl3 (12 mL). Subsequently,
the solution was stirred under reflux for 16 h. The solvent was then
removed under vacuum, and the remaining solid residue washed
three times with 10 mL portions of Et2O. The solid residue was
found C 56.18, H 9.17, N 33.94. 1H NMR (399.89 MHz, CD3CN): then dried and redissolved in CH3CN (6 mL). The solution was
δ = 5.86 (s, 1 H, HAr), 2.64 (s, 24 H, CH3), 2.62 (s, 24 H, CH3) slowly added at 0 °C to a suspension of 2,3,5,6-tetrakis(amino)-
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S. Stang, A. Lebkücher, P. Walter, E. Kaifer, H.-J. Himmel
FULL PAPER
benzoquinone (0.100 g, 0.5947 mmol) in CH3CN (6 mL). The reac-
tion mixture was gradually warmed up from 0 °C to room tempera-
ture over a period of 5 h. The solvent was then removed under
vacuum, and the remaining residue dissolved in H2O (5 mL). A
NaOH solution (7 mL of a 25% solution) was added to this green–
brown solution, which led to a deep green solution. The solution
was then extracted with CH3CN, until the organic phase remained
colourless. The combined organic phases were dried with K2CO3,
and the solvent removed under vacuum to yield the crude product
to 29.99°. Reflections measured 10807, independent 5509, Rint
0.0269. Final R indices [IϾ2σ(I)]: R1 = 0.0410, wR2 = 0.1007.
=
[3H4]Cl4: Compound 3 (10.0 mg, 0.018 mmol) was dissolved in
CH3CN (2 mL). A solution of HCl in Et2O (0.4 mL of a 0.2 m
solution, 0.08 mmol) was then added, and the colour of the solu-
tion changed from green to red. The reaction mixture was stirred
at room temperature for 1 h. Subsequently, the solution was con-
1
centrated and stored at 0 °C to afford dark red crystals. H NMR
(200 MHz, CD3CN): δ = 10.61 (s, 4 H, NH), 3.04 (s, 48 H, CH3)
(300.7 mg, 0.54 mmol, 91%) in the form of
a green solid.
ppm. Crystal data for C26H72Cl4N12O12: Mr
=
886.76,
Recrystallization from a CH3CN/C7H8 mixture gave crystals of 4
0.20ϫ0.16ϫ0.16 mm, monoclinic, space group P21/n,
a
=
suitable for an XRD analysis. C26H40N12O2 (552.70): calcd. C
11.128(2) Å, b = 15.468(3) Å, c = 13.767(3) Å, β = 108.30(3)°, V =
2249.8(9) Å3, Z = 2, dcalcd. = 1.309 Mgm–3, Mo-Kα radiation
(graphite monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.04
to 27.48°. Reflections measured 10259, independent 5145, Rint
0.0370. Final R indices [IϾ2σ(I)]: R1 = 0.0437, wR2 = 0.1124.
1
56.50, H 7.29, N 30.41; found C 54.95, H 7.37, N 29.56. H NMR
(600.13 MHz, CD3CN): δ = 3.20 [dd (m), J = 16.94, J = 4.23 Hz,
16 H, CH2], 2.65 (s, 24 H, CH3) ppm. 13C{1H} NMR (150.90 MHz,
CD3CN), HSQC, HMBC: δ = 183.70 (s, C=O), 155.67 (s, NCN),
134.68 (s, Cring), 48.79 (t, CH2), 34.23 (q, CH3) ppm. 1H NMR
(600.13 MHz, CD2Cl2): δ = 3.23 (m, 16 H, CH2), 2.70 (s, 24 H,
CH3) ppm. 13C{1H} NMR (150.90 MHz, CD2Cl2), HSQC,
HMBC: δ = 183.38 (s, C=O), 155.62 (s, NCN), 134.21 (s, Cring),
48.72 (t, CH2), 34.33 (q, CH3) ppm. MS (HR-EI+): m/z (%) =
552.3420 (100) [M]+ (calcd. 552.3397), (9.02) [M]2+ (calcd.
=
[(2-Br)H](Br3)2: Compound 2 (30.0 mg, 0.056 mmol) was dissolved
in CH3CN (6 mL). Then Br2 (8.69 μL, 27.0 mg, 0.17 mmol) was
then added to the pale green solution. Brown crystals precipitated
from the brown reaction mixture after addiction of toluene (6 mL).
Yield: 38.1 mg, 0.028 mmol (51% relative to 2, 83% relative to bro-
mine). C25H49Br10N13 (1330.78): calcd. C 22.56, H 3.71, N 13.68;
found C 23.02, H 3.89, N 14.10. 1H NMR (399.89 MHz, CD3CN):
δ = 8.39 (s, 1 H, NH), 3.13 (s, 12 H, CH3), 3.02 (s, 24 H, CH3),
2.98 (s, 12 H, CH3) ppm. 13C{1H} NMR (100.56 MHz, CD3CN),
HSQC, HMBC: δ = 41.87 (q, CH3) ppm. MS (ESI+): m/z (%) =
687.9 (80.11) [2-Br]2+Br–, 304.8 (100.0) [2-Br]2+. UV/Vis (Et2O): λ
(ε, Lmol–1 cm–1) = 360 (11870), 269 (82736) nm. Crystal data for
C25H49Br10N13: Mr = 1330.77, 0.35ϫ0.35ϫ0.30 mm, monoclinic,
space group P21/c, a = 21.190(4) Å, b = 13.057(3) Å, c =
276.1761). IR (CsI): ν = 2940 (w), 2862 (w), 2831 (w), 1655 (s),
˜
1597 (m), 1493 (w), 1458 (w), 1443 (w), 1412 (w), 1393 (w), 1335
(w), 1315 (m), 1288 (m), 1238 (m), 1192 (w), 1169 (w), 1138 (w),
1122 (w), 1080 (w), 1030 (m), 999 (m), 984 (w), 968 (m), 949 (s),
860 (w), 802 (w), 768 (w), 748 (w), 706 (w), 671 (w), 652 (w), 633
(w), 610 (w), 579 (w), 532 (w), 505 (w) cm–1. UV/Vis (CH3CN): λ
(ε, Lmol–1 cm–1) = 400 (14237), 277 (28614) nm. Crystal data for
C26H40N12O2: Mr = 552.70, 0.50ϫ0.40ϫ0.40 mm, monoclinic,
space group P21/c,
a
=
9.839(2) Å,
b
=
13.628(3) Å,
c
=
=
17.766(4) Å, β = 112.67(3)°, V = 4536(2) Å3, Z = 4, dcalcd.
=
10.650(2) Å, β = 93.31(3), V = 1425.6(5) Å3, Z = 2, dcalcd.
1.949 Mgm–3, Mo-Kα radiation (graphite monochromated, λ =
0.71073 Å), T = 100 K, θrange 2.08 to 33.14°. Reflections measured
1.288 Mgm–3, Mo-Kα radiation (graphite monochromated, λ =
0.71073 Å), T = 100 K, θrange 2.43 to 30.03°. Reflections measured
8134, independent 4163, Rint = 0.0315. Final R indices [IϾ2σ(I)]:
R1 = 0.0464, wR2 = 0.1183.
32211, independent 17183, Rint
= 0.0455. Final R indices
[IϾ2σ(I)]: R1 = 0.0423, wR2 = 0.0823.
[2-I](I3)I: Compound 2 (30.3 mg, 0.057 mmol) was dissolved in
CH3CN (5 mL), and I2 (28.9 mg, 0.11 mmol) added to the pale
green solution. The reaction mixture turned deep red. Red crystals
were obtained after addition of toluene (5 mL). C25H48I5N13·0.5
toluene (1211.32): calcd. C 28.26, H 4.34, N 15.03; found C 28.50,
2-(β-Hydroxy-ethylamino)-p-benzoquinone: The compound was syn-
thesized as described in the literature (yield: 70%)[26] and crys-
tallized from MeOH solutions. Crystal data for C10H13NO4: Mr =
422.43, 0.50ϫ0.50ϫ0.40 mm, monoclinic, space group P21/n, a =
8.4980(17) Å, b = 12.451(3) Å, c = 8.9990(18) Å, β = 93.14(3)°, V
= 950.7(3) Å3, Z = 2, dcalcd. = 1.476 Mgm–3, Mo-Kα radiation
(graphite monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.91
1
H 4.34, N 15.06. H NMR (399.89 MHz, CD3CN): δ = 2.92 (s, 24
H, CH3), 2.91 (s, 24 H, CH3) ppm. 13C{1H} NMR (100.56 MHz,
CD3CN), HSQC, HMBC: δ = 169.91 (s, NCN), 167.54 (s, NCN),
155.85 (s), 154.52 (s), 83.26 (s, CI), 41.60 (q, CH3), 41.13 (q, CH3)
ppm. MS (ESI+): m/z (%) = 657.5 (16.11) [2-I]+, 531.3 (10.51) [2]+,
328.7 (23.01) [2-I]2+, 265.8 (100) [2]2+. UV/Vis (Et2O): λ (ε,
Lmol–1 cm–1) = 513 (7870), 366 (39795), 290 (38726), 245 (54800)
to 27.91°. Reflections measured 2546, independent 1789, Rint
0.0377. Final R indices [IϾ2σ(I)]: R1 = 0.0486, wR2 = 0.1211.
=
[2H4]Cl4: Compound 2 (60.0 mg, 0.11 mmol) was dissolved in Et2O
(10 mL). To this pale green solution, HCl in EtOH (0.4 mL of a
1 m solution) was added. A pale yellow solid precipitated from the
cloudy solution. The solution was stirred for 30 min at room tem-
perature, and the solvent was then removed under vacuum. The
remaining colourless residue was crystallized from hot CH3CN
(3 mL) to yield colourless crystals of [2H4]Cl4·2H2O (58.9 mg,
0.083 mmol, 73%). 1H NMR (399.89 MHz, CD3CN): δ = 11.72 (s,
2 H, NH), 11.33 (s, 2 H, NH), 7.43 (s, 1 H, HAr), 3.03 (s, 24 H,
CH3), 3.00 (s, 24 H, CH3) ppm. 13C{1H} NMR (100.56 MHz,
CD3CN), HSQC, HMBC: δ = 159.37 (s, NCN), 158.90 (s, NCN),
nm. Crystal data for C32H56I5N13: Mr
=
1257.40,
0.30ϫ0.15ϫ0.15 mm, monoclinic, space group Cc, a = 15.983(3),
b = 15.896(3), c = 17.995(4) Å, β = 97.28(3)°, V = 4535.1(16) Å3,
Z = 4, dcalcd. = 1.842 Mgm–3, Mo-Kα radiation (graphite mono-
chromated, λ = 0.71073 Å), T = 100 K, θrange 2.28 to 32.09°. Reflec-
tions measured 48382, independent 15614, Rint = 0.0457. Final R
indices [IϾ2σ(I)]: R1 = 0.0389, wR2 = 0.0956.
[2-I]I2: Compound 2 (30.3 mg, 0.057 mmol) was dissolved in
141.42 (s, CAr), 129.09 (s, CAr, CH), 123.34 (d, CAr), 41.71 (q, CH3) CH2Cl2 (5 mL), and I2 (28.9 mg, 0.11 mmol) added to the pale
ppm. MS(ESI+): m/z (%) = 532.3 (60.11) [2H]+, 266.7 (100.0)
[2H2]+. Crystal data for [2H4]Cl4·2H2O, C25H57Cl4N13O2: Mr
713.64, 0.20ϫ0.20ϫ0.15 mm, monoclinic, space group P21/n, a =
green solution. The reaction mixture turned deep red. Red crystals
were obtained after addition of Et2O (5 mL). 1H NMR
(600.13 MHz, CD3CN): δ = 2.93 (s, 24 H, CH3), 2.91 (s, 24 H, CH3)
=
8.2090(16), b = 25.283(5), c = 9.769(2) Å, β = 111.29(3), V = ppm. 13C{1H} NMR (150.90 MHz, CD3CN), HSQC, HMBC: δ
1889.2(8) Å3, Z = 2, dcalcd. = 1.255 Mgm–3, Mo-Kα radiation = 169.88 (s, NCN), 167.52 (s, NCN), 155.83 (s), 154.49 (s), 83.31
(graphite monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.38
(s, CI), 41.64 (q, CH3), 41.15 (q, CH3) ppm. Crystal data for [2-I]
10
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20679
/KAP1
Date: 05-09-12 17:05:05
Pages: 14
Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
I2·CH2Cl2, C26H50Cl2I3N13: Mr = 996.39, 0.30ϫ0.25ϫ0.25 mm,
tetragonal, space group P4(3), a = 11.4730(16), b = 11.4730(16), c
dissolved by briefly heating. Pale green crystals were isolated from
the deep green reaction mixture. Yield: 70.2 mg, 0.09 mmol, 66%.
= 33.024(7) Å, V = 4346.9(12) Å3, Z = 4, dcalcd. = 1.522 Mgm–3, C25H49Br2Cu2N13 (818.64): calcd. C 36.68, H 6.03, N 22.24; found
1
Mo-Kα radiation (graphite monochromated, λ = 0.71073 Å), T =
C 35.70, H 5.99, N 21.28. H NMR (600.13 MHz, CD3CN): δ =
100 K, θrange 2.47 to 27.45°. Reflections measured 9845, indepen- 5.87 (s, 1 H, HAr), 2.78 (s, 48 H, CH3) ppm. 13C{1H} NMR
dent 9840, Rint = 0.0417. Final R indices [IϾ2σ(I)]: R1 = 0.0606,
wR2 = 0.1704.
(150.90 MHz, CD3CN): δ = 163.26 (s), 40.17 (q, CH3) ppm. MS
(FAB+): m/z (%) = 819.2 (26.00) [M]+, 738.2 (68.00) [M – Br]+,
676.2 (100.0) [M – CuBr + H]+. IR (CsI): ν = 2995 (w), 2933 (w),
˜
(3)I2: Compound 3 (16.8 mg, 0.030 mmol) was dissolved in CH2Cl2
(5 mL), and I2 (7.60 mg, 0.030 mmol) was added. The colour of the
solution turned from dark green to dark blue. Black crystals with
a red metallic shine were obtained by layering the solution with
Et2O (5 mL). C26H48I2N12O2·3CH2Cl2 (1069.35): calcd. C 32.57, H
5.09, N 15.72; found C 32.32, H 5.07, N 16.36. 1H NMR
(600.13 MHz, CD3CN): δ = 2.91 (s, CH3) ppm. 13C{1H} NMR
(150.90 MHz, CD3CN), HSQC, HMBC: δ = 171.68 (s, NCN),
168.10 (s, C=O), 149.81 (s, Cring), 41.12 (q, CH3) ppm. MS (HR-
ESI+): m/z (%) = 687.30641 (39.4) [M – I]+ (calcd. 687.30680),
560.40185 (82.7) [M – I2]+ (calcd. 560.40177), 280.20063 (100.0)
2876 (w), 2792 (w), 1616 (m), 1541 (m), 1388 (m), 1151 (s), 1022
(vs), 896 (s), 719 (s) cm–1. Crystal data for C33H61Br2Cu2N17: Mr
= 982.89, 0.40ϫ0.30ϫ0.25 mm, monoclinic, space group C2/c, a
= 20.121(4), b = 12.566(3), c = 20.279(4) Å, β = 116.55(3)°, V =
4587(2) Å3, Z = 4, dcalcd. = 1.423 Mgm–3, Mo-Kα radiation (graph-
ite monochromated, λ = 0.71073 Å), T = 100 K, θrange 2.37 to
30.00°. Reflections measured 25088, independent 6660, Rint
=
0.0774. Final R indices [IϾ2σ(I)]: R1 = 0.0673, wR2 = 0.1614 cm–1.
[2(CuI)2]: Compound 2 (30 mg, 0.056 mmol) was dissolved in
CH3CN (8 mL). CuI (21.5 mg, 0.113 mmol) was then added, and
the reaction mixture stirred at room temperature for 30 min. A pale
green precipitate formed, which was redissolved by briefly heating.
Pale green crystals were isolated from the deep green reaction mix-
ture. Yield: 48.0 mg, 0.052 mmol, 93%. C25H49Cu2I2N13 (912.64):
calcd. C 32.90, H 5.41, N 19.95; found C 32.74, H 5.41, N 19.30.
1H NMR (199.92 MHz, CD2Cl2): δ = 5.75 (s, 1 H, HAr), 2.80 (s,
48 H, CH3) ppm. MS (FAB+): m/z (%) = 913.1 (6.896) [M]+, 784.6
(26.21) [M – I]+, 722.4 (25.51) [M – CuI]+, 594.3 (13.79) [M – CuI –
[M – I2]2+ (calcd. 280.20061). IR (CsI): ν = 2997 (w), 2957 (m),
˜
2917 (m), 2797 (w), 1599 (s), 1559 (m), 1504 (vs), 1448 (m), 1397
(vs), 1357 (m), 1292 (m), 1320 (w), 1171 (m), 1062 (s), 1000 (w),
898 (m), 797 (w), 768 (m), 727 (m), 688 (w), 656 (m), 608 (w),
569 (w), 549 (w), 494 (w), 473 (w), 458 (w), 419 (m) cm–1. UV/Vis
(CH3CN): λ (ε, Lmol–1 cm–1) = 581 (33284), ca. 535 (24720), ca.
378 (3657), 282 (16476), 246 (33171), 209 (63756) nm. Crystal data
for
(3)I2·2CH2Cl2,
C28H52Cl4I2N12O2:
Mr
=
984.42,
¯
0.20ϫ0.20ϫ0.12 mm, triclinic, space group P1, a = 9.7340(19), b
= 10.415(2), c = 10.673(2) Å, α = 106.70(3)°, β = 91.28(3)°, γ =
100.37(3)°, V = 1016.3(4) Å3, Z = 1, dcalcd. = 1.608 Mgm–3, Mo-
Kα radiation (graphite monochromated, λ = 0.71073 Å), T = 100 K,
θrange 2.00 to 29.00°. Reflections measured 9567, independent 5317,
I]+, 531.5 (100.0) [2]+. IR (CsI): ν = 3001 (w), 2936 (w), 2881 (w),
˜
2794 (w), 1622 (m), 1558 (vs), 1521 (vs), 1448 (s), 1421 (vs), 1389
(vs), 1153 (vs), 1024 (vs), 898 (m), 707 (m) cm–1. Crystal data for
C33H61Cu2I2N17: Mr = 1076.89, 0.30ϫ0.25ϫ0.25 mm, mono-
clinic, space group C2/c, a = 20.385(4), b = 12.663(3), c =
Rint = 0.0377. Final R indices [IϾ2σ(I)]: R1 = 0.0387, wR2
=
20.483(4) Å, β = 117.02(3)°, V = 4710(2) Å3, Z = 4, dcalcd.
=
0.0769.
1.519 Mgm–3, Mo-Kα radiation (graphite monochromated, λ =
0.71073 Å), T = 100 K, θrange 2.23 to 30.09°. Reflections measured
40967, independent 6901, Rint = 0.0638. Final R indices [IϾ2σ(I)]:
R1 = 0.0388, wR2 = 0.0975.
(4)I2: Compound 4 (21.0 mg, 0.038 mmol) was dissolved in CH2Cl2
(5 mL). To this green solution was added I2 (9.64 mg, 0.038 mmol).
The solution turned purple immediately. Black crystals with a red
metallic shine were obtained by layering the solution with Et2O
(5 mL). C26H40I2N12O2·CH2Cl2 (891.42): calcd. C 36.38, H 4.75, N
18.86; found C 35.12, H 4.77, N 17.53. MS (HR-ESI+): m/z (%) =
679.24393 (100.0) [M – I]+ (calcd. 679.24420), 552.33955 (41.9)
[2(CuCN)2]: Compound 2 (50 mg, 0.09 mmol) was dissolved in
CH3CN (8 mL). CuCN (16.8 mg, 0.19 mmol) was added, and the
pale green reaction mixture was heated at reflux for 30 min. The
reaction mixture was cooled to room temperature overnight; a yel-
low precipitate formed, which was dissolved by briefly heating. Yel-
low crystals suitable for an XRD analysis were isolated from the
pale green reaction mixture. Yield: 47.3 mg, 0.07 mmol, 71%.
C27H49Cu2N15 (710.87): calcd. C 45.62, H 6.95, N 29.56; found C
1
[M – I2]+ (calcd. 552.33972). H NMR (600.13 MHz, CD3CN): δ
= 3.75 (s, 16 H, CH2), 2.70 (s, 24 H, CH3) ppm. 13C{1H} NMR
(150.90 MHz, CD3CN), HSQC, HMBC: δ = 168.53 (s, C=O),
168.05 (s, NCN), 152.22 (s, Cring), 49.16 (t, CH2) 32.98 (q, CH3)
ppm. IR (CsI): ν = 2963 (w), 2925 (w), 2886 (w), 2806 (w), 1570
˜
1
44.52, H 6.74, N 28.69. H NMR (600.13 MHz, CD2Cl2): δ = 5.70
(s), 1560 (s), 1558 (s), 1491 (s), 1477 (s), 1419 (m), 1368 (m), 1348
(m), 1292 (m), 1262 (w), 1235 (w), 1205 (w), 1072 (s), 1018 (m),
965 (m), 875 (w), 798 (m), 765 (w), 733 (w), 680 (w), 646 (w), 609
(w), 581 (w), 537 (w), 502 (w), 419 (w) cm–1. UV/Vis (CH3CN): λ
(ε, Lmol–1 cm–1) = 575 (38556), ca. 522 (24269), 365 (6591), 285
(18604), 247 (29109) nm. Crystal data for (4)I2, C30H46I2N14O2:
(s,
1
H, HAr), 2.83 (s, 48 H, CH3) ppm. 13C{1H} NMR
(150.90 MHz, CD2Cl2), HSQC, HMBC, DEPT: δ = 165.37 (s,
NCN), 162.74 (s, NCN), 147.83 (s, CAr), 129.99 (s, CAr), 115.81 (s,
CAr,CH), 40.28 (q, CH3) ppm. MS (FAB+): m/z (%) = 709.9 (26.00)
[M]+, 685.6 (30.00) [M – CN + H]+, 621.5 (35.00) [M – CuCN +
H]+, 594.5 [M – CuCN – CN]+, 531.6 [2]+. IR (CsI): ν = 3003 (w),
˜
¯
Mr = 888.61, 0.70ϫ0.50ϫ0.50 mm, triclinic, space group P1, a =
2937 (w), 2882 (w), 2797 (w), 2103 (s), 1523 (vs), 1464 (s), 1424
(vs), 1392 (vs), 1246 (s), 1230 (s), 1156 (vs), 1027 (vs), 899 (s), 719
(s) cm–1. UV/Vis (CH3CN): λ (ε, Lmol–1 cm–1) = 407 (17922), 331
(24609), 224 (56502) nm. Crystal data for [2(CuCN)2]·4CH3CN,
C35H61Cu2N19: Mr = 875.13, 0.30ϫ0.20ϫ0.20 mm, monoclinic,
space group C2/c, a = 20.211(4), b = 12.549(3), c = 20.016(4) Å, β
= 115.68(3)°, V = 4575(2) Å3, Z = 4, dcalcd. = 1.271 Mgm–3, Mo-
9.776(2), b = 10.030(2), c = 10.209(2) Å, α = 90.82(3)°, β =
109.23(3)°, γ = 93.73(3)°, V = 942.5(3) Å3, Z = 1, dcalcd.
=
1.566 Mgm–3, Mo-Kα radiation (graphite monochromated, λ =
0.71073 Å), T = 100 K, θrange 2.88 to 35.00°. Reflections measured
14600, independent 8163, Rint = 0.0335. Final R indices [IϾ2σ(I)]:
R1 = 0.0370, wR2 = 0.0925.
[2(CuBr)2]: Compound 2 (71.7 mg, 0.13 mmol) was dissolved in Kα radiation (graphite monochromated, λ = 0.71073 Å), T = 100 K,
CH3CN (8 mL). CuBr (38.7 mg, 0.27 mmol) was then added to the θrange 2.50 to 27.55°. Reflections measured 36321, independent
pale green solution. The reaction mixture was stirred at room tem-
perature for 30 min. A pale green precipitate formed, which was
5234, Rint = 0.0494. Final R indices [IϾ2σ(I)]: R1 = 0.0452, wR2
= 0.1084.
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
11

Job/Unit: I20679
/KAP1
Date: 05-09-12 17:05:05
Pages: 14
S. Stang, A. Lebkücher, P. Walter, E. Kaifer, H.-J. Himmel
FULL PAPER
[(2-I)(CuI)2](I3)2]n: I2 (43.2 mg, 0.23 mmol) was added to a suspen-
sion of [2(CuI)2] (74.9 mg, 0.08 mmol) in CH3CN (8 mL). The col-
our of the reaction mixture turned immediately from dark green to
black. The reaction mixture was stirred for 30 min, after which a
black solid precipitated. The black solution was removed by sy-
ringe, and the precipitate was washed with CH3CN. Yield (raw
product): 112 mg, 76%, 0.06 mmol. C25H49Cu2I8N13 (1674.07):
calcd. C 17.94, H 2.95, N 10.88; found C 18.62, H 3.05, N 11.26.
MS (ESI+): m/z (%) = 531.3 (4.85) [2]+, 265.8 (100.0) [2]2+. MS
All structures were optimized by using the structures derived exper-
imentally in the crystalline phase as starting points if available, and
vibrational frequencies were calculated. The calculations on the
ΔG0 values for the gas-phase redox reactions involving 1a, 2, 3 and
4 relied on the B3LYP functional[33] and the 6-311G** basis set.[34]
The pKa values in CH3CN solutions were estimated according to
the procedure of Maksic´ et al.[24] First, the structures of the neutral
and the protonated forms were optimized by using the B3LYP
functional together with the 6-311G** basis set. Finally, the ener-
gies were retrieved from a B3LYP/6-311+G** calculation.[33b,35]
The calculations used the conductor-like polarizable continuum
model to describe solvation by CH3CN.
(ESI–): m/z (%) = 381.0 (40.68) [I ]–, 127.2 (100.0) [I]–. IR (CsI): ν
˜
3
= 3013 (w), 2964 (w), 2829 (w), 2803 (w), 1617 (s), 1551 (s), 1510
(s), 1482 (vs), 1460 (vs), 1401 (vs), 1387 (vs), 1292 (vs), 1227 (w),
1173 (ss), 1064 (w), 1030 (m), 894 (w), 795 (w) cm–1. UV/Vis
(CH3CN): λ (ε, Lmol–1 cm–1) = 502 (4153), 364 (33517), 291
(46050), 245 (43998) nm.
Supporting Information (see footnote on the first page of this arti-
cle): Crystal structure of the benzoquinone derivative formed by
elimination of two dimethylamino groups, UV/Vis spectrum of 2,
molecular structure of 2-(β-hydroxyethylamino)-p-benzoquinone,
absorption and emission spectra of 2-(β-hydroxyethylamino)-p-
benzoquinone, calculated structures and Gibbs free energy differ-
ences for two possible tautomers of [2H]+ and [3H]+ and fraction
of the crystal structure of [2-I]I2 are presented.
[2(CuI)2] + CH3I: Methyliodide (14.9 μL, 34.0 mg, 0.24 mmol) was
slowly added to a dark green suspension of [2(CuI)2] (49.0 mg,
0.054 mmol) in acetone (3 mL) at room temperature. The reaction
mixture was stirred further for 16 h at room temperature, but no
reaction occurred. In another experiment methyliodide (6.65 μL,
15.2 mg, 0.11 mmol) was slowly added to a dark green suspension
of [2(CuI)2] (81.6 mg, 0.09 mmol) in acetonitrile (3 mL) at room
temperature. The reaction mixture was heated to 48 °C for 8 h, but Acknowledgments
again no reaction occurred.
The authors gratefully acknowledge continuous financial support
by the Deutsche Forschungsgemeinschaft (DFG).
[3(CuI)(CuI2)I]n: CuI (13.58 mg, 0.07 mmol) was added to a solu-
tion of 3 (20 mg, 0.036 mmol) in CH3CN (5 mL). The solution
turned brown, and after 1 h, the solvent was removed. Black crys-
tals could be obtained by slow evaporation of the solvent. Crystal
[1] a) C. Wurster, Ber. Dtsch. Chem. Ges. 1879, 12, 522–528; b) C.
Wurster, E. Schobig, Ber. Dtsch. Chem. Ges. 1879, 12, 1807.
[2] a) K. Elbl, C. Krieger, H. A. Staab, Angew. Chem. 1986, 98,
1024–1026; Angew. Chem. Int. Ed. Engl. 1986, 25, 1023–1025;
b) K. Elbl-Weiser, F. A. Neugebauer, H. A. Staab, Tetrahedron
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¯
0.25ϫ0.15ϫ0.15 mm, triclinic, space group P1, a = 13.057(3), b =
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plementary publication nos. CCDC-843459 (2), -843455 (3),
-843457 (4), -843458 [2-(β-hydroxyethylamino)-p-benzoquinone],
843461 ([2H4]Cl4), 843456 ([3H4]Cl4), 848206 ([(2-Br)H](Br3)2),
843460 ([2-I](I3)I), 843462 ([2-I]I2), 843454 [(3)I2], 886681 [(4)I2],
843464 ([2(CuBr)2]), 843463 ([2(CuI)2]), 886682 [2(CuCN)2],
886680 [3(CuI)(CuI2)I]n contain the supplementary crystallo-
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Redox-Active Guanidine Ligands with Pyridine and p-Benzoquinone Backbones
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Received: June 21, 2012
Published Online: ᭿
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
13

Job/Unit: I20679
/KAP1
Date: 05-09-12 17:05:05
Pages: 14
S. Stang, A. Lebkücher, P. Walter, E. Kaifer, H.-J. Himmel
FULL PAPER
Redox-Active Ligands
Three new members of the family of redox-
active guanidine ligands are presented, and
their redox potential as well as aspects of
their coordination chemistry studied.
S. Stang, A. Lebkücher, P. Walter,
E. Kaifer, H.-J. Himmel* ................ 1–14
Redox-Active Guanidine Ligands with
Pyridine and p-Benzoquinone Backbones
Keywords: Guanidine / Redox chemistry /
Coordination compounds / Copper / Den-
sity functional calculations
14
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Eur. J. Inorg. Chem. 0000, 0–0